The Laser Interferometer Gravitational-Wave Observatory (LIGO), which first detected gravitational waves from colliding black holes in 2016, and the European Virgo gravitational-wave detector, which observed the neutron star merger-produced waves in October, both detect objects in the moments right before they merge. Supermassive black holes have a mass billions of times than that of the sun. These observatories assemble their perception by disseminating lasers down long tunnels and exactly calculating minute deformities in the beam.

"Observing low-frequency gravitational waves would be akin to being able to hear bass singers, not just sopranos", said Joseph Lazio, chief scientist of Nasa's Deep Space Network and co-author of the study. Pulsars are dense neutron stars that rapidly rotate, emitting electromagnetic signals like clockwork.

With enough data, by studying the periodicity and strength of the pulsar signals, scientists can infer the presence and strength of a gravitational wave if the pulsar signal is delayed even by a tiny amount. These waves, being much stronger, have a far lower frequency than the gravitational waves that have been observed so far, which is why they can not be detected by human-made instruments. Importantly, this method could be more tuned towards lower frequency waves.

"A difference between when the pulsar signals should arrive, and when they do arrive, can signal a gravitational wave", says Chiara Mingarelli, lead author of the new study and a research fellow at the Center for Computational Astrophysics at the Flatiron Institute in New York City.

The detectors have administered to pick up ripples from black holes and neutron stars with masses multiple times greater than the Sun but waves from extensive cosmic cataclysms like smashup between two galaxies have so far avoided detection. "And since the pulsars we study are about 3,000 light-years away, they act as a galactic-scale gravitational-wave detector", she added.

Ultimately, detecting a gravitational wave generated by merging supermassive black holes could teach us more about how galaxies form and merge, helping us unravel the secrets of the universe.

Of the 5,000 galaxies they studied, the astronomers narrowed in on about 90 that probably have pairs of supermassive black holes in the process of merging. Smaller mergers meanwhile can happen over more than a hundred million years.

The researchers hope that the array can also teach us about how galaxies are formed and what happens when they merge - which could be useful information, considering we're now on-course for a collision with our neighboring Andromeda galaxy.

"By expanding our pulsar timing array over the next 10 years or so, there is a high likelihood of detecting gravitational waves from at least one supermassive black hole binary", Mingarelli said.